Buffer Control Logic - any problems with this?

Brian_18

I'm adding a buffer tank to my system to help cut down short cycling. System consists of a mod-con piped P/S, with a mixture of emitter zones of both fin tube baseboard, and in-floor radiant (some constant circ.). The radiant runs on it's own P/S loop at a lower temp via injection. Both the radiant mixing & boiler are under reset control. I plan to have the injection pump draw only from the buffer tank (50 gal), and have it charged as a secondary load on the high temp loop. My control strategy is the buffer charge circulator will run any time a fin tube zone is calling. The tank will also have it's own setpoint controller, which can call to charge anytime the setpoint calls. I will have the setpoint call for a DHW priority function, which will overide the boiler reset, and charge the buffer to maximum. So, if the buffer calls on it's own, the boiler will go to DHW setpoint, charge the buffer then shutdown. If any fintube zone is calling, the buffer will charge to the boiler reset temp, then after the fintube call is satisfied, the buffer will continue to call by it's setpoint control which will ramp the boiler to DHW temp, then shut down. Anyway, that's my logic. Does this make sense? Any problems with this logic?

jp_2

what if you

ran your fintub return into your in-floor heat thus lowering your return-to-boiler temp lengthening your cycles?

sounds a lot like a system I am planning. boiler will charge the buffer. buffer will feed all systems through a mixing valve.

added: you can always double up areas, wait for 2 zones to call before boiler cycles.

Mike T., Swampeast MO

Why the short-cycling problem? Oversized mod-con? Deep daily setback with extremely generous reset curve(s)?

hot_rod

So the buffer

is held at a high temperature and and it only supplies low temperature radiant via injection mixing?

What are the various loads in BTU/hr? Which load is causing the short cycling? You may fix the short cycling but add some operating cost by keeping the buffer at a high temperature?

hot rod

Brian_18

Answers

Yes, my boiler is currently oversized. It was sized to handle rooms over my new garage which will not be completed for a couple years, and also to handle heating the basement, which will not be done for a while either. None the less, the boiler turndown can act friendly during most of the heating season. I only have short cycling during lead-in and tail-out of the heating season, maybe a month on either side. The buffer tank will be a short term (2 yr) solution until the boiler is loaded to ALL the load it was sized for. A buffer tank seemed to be an inexpensive way to help cut down on needless wear & tear (and maybe give economy) on the boiler by cutting down on the short cycling during light load months. That said, my question was to know of any pittfalls to my proposed control strategy. The buffer will be piped secondary to the boiler primary. The injection pump will draw exclusively from the buffer. I don't have an indirect DHW, and don't plan on one. I assumed it would extend re-charge times by packing more BTU's into the tank. I thought an elegant way to handle this would be to have the buffer charge to reset temp. any time a fin tube zone is calling, then only after the fin tube ceases, the buffer calls for DHW setpoint temp which boosts the BTU's in the buffer. Hot Rod indicates this may be a waste of energy, if that's true I can choose to scrap that idea. I built the buffer last night, and plan on installing today. I'll report back on how it works at a later date. As always, thanks very much for exchanging ideas, AND for not shunning DIY'ers as myself.

Unknown

why not just dump the boiler primary circuit into the buffer tank, and pull your baseboard/injection off of the other side?

charging up the buffer tank with a DHW call is a waste of the modulating ability of the boiler as well as its condensing ability. You size the buffer to provide enough charge/draw down time based on the temperature range you're cycling the boiler on, and the min mod level of the boiler. Then regardless of the temp it is reset to, you still are charging up over the same temperature differential from start to finish.

No reason to make buffer tank "temporary". You're always going to have portions of the heating season under your min mod level, if you have a buffer in, you might as well leave it even if it becomes less necessary later. The only reason not to have a buffer on every system with small zones is economic... if it's already there, that arguement is gone.